The reason they are cheap is because they aren't worth much. With one drawing they sent me one of a ship (De Ruyter) from a totally different era (what's in a name). Fortunately, there are two happy options
http://www.modelbouwers.nl/ have a large collection of drawings that are slightly less cheap but far more worthwhile (site offline at the time of posting). I bought a few decent plansets from them.
The Ministry of Defense's archives contain the original construction drawing on Microfilm and you can get very cheap reproductions. Unfortunately, in order to get the drawing on microfilm, they cut up all larger drawings into parts! The barbarians! So you need to spend some time stitching them together. I have a series of to-be-stitched up De Ruyter plans and a nice set of all boats and launches I deemed relevant to own (a lot!). Now, the De Ruyter plan set is beyond email size but I can send scans of the boats and launches to people who are can convince me they need them
For other drawings you have to visit them.Meanwhile: visit Maritiem Digital (Maritime Digital), a website showing images from all Dutch maritime musea. For the De Ruyter there are 600 images!